This invention relates in general to servo control systems and, in particular, to incremental motion systems.
In document processing systems it has become desirable to film moving documents in order to more permanently store the information contained therein. In order to make such filming economical, these documents, which can be as much as 9.25 inches long, flow through the system at a rate of up to 600 documents per minute. Therefore, it is a system requirement that the documents be filmed while the documents are moving at approximately 100 inches per second. Patent application Ser. No. 723,361, filed Sept. 15, 1976 and entitled "Document Photography System", now U.S. Pat. No. 4,087,173, and claims a document photography system which can maintain this through-put level while a particular document is recorded on the film with a 50:1 reduction. To achieve the required resolution on the film the lens shuttle of the subject document photography system must move during the filming process at a velocity that is proportional to the velocity of the document and thereafter return to its initial position to film a second moving document. Furthermore, the velocity should be maintained to a high degree of accuracy to minimize the resolution problems on the film. A phase-locked servo system having different modes of operation was therefore required to provide the unique combination of incremental motion drive and high precision velocity control. In particular, the phase-locked servo system was required to respond within typically 10 milliseconds of a reference signal indicating the movement of a document through the system.
A first apparatus is described and claimed for controllably moving a load from a first position to a second position in a forward mode to track a moving object field in response to an external reference signal. An apparatus constructed in accordance with the invention comprises a driving means responsive to the reference signal for moving the load out of the first position towards the second position. The driving means includes a motor for supplying the moving force. The load has a predetermined displacement in the second position. The apparatus also includes sensing means for sensing the displacement of the load by generating a plurality of pulses at predetermined intervals along the displacement. The pulses generated in the forward mode define a feedback signal having a corresponding frequency. Also included is a forward control circuit means responsive to the reference signal and the feedback signal in the forward mode to provide a forward control signal. The forward control circuit means includes a rate comparator means which is responsive to the reference signal and the sensing means to provide the driving means with a velocity signal which corresponds to the discrepency between the reference signal and the resulting feedback signal.
A second apparatus is described and claimed for controllably moving the load back to the first position from the second position in a return mode. A second apparatus constructed in accordance with the invention includes the driving means of the first apparatus for moving the load out of the second position towards the first position. Also included is the sensing means of the first apparatus which generates a plurality of signals or pulses at the same predetermined intervals as the load moves from the second position to the first position in the return mode. A control means which is responsive to the difference in the number of pulses in the forward mode and the return mode and which is responsive to the return angular velocity provides a return control signal. The driving meams is responsive to the return control signal to accelerate and decelerate the load in alternating fashion.